It is well known that vascular stiffness increases with aging, and that the effects of aging on arterial stiffness are relatively protected in older women. Although most prior mechanistic work on the effects of aging on vascular regulation and stiffness has been conducted in rodent models, the extent to which these data can be extrapolated to humans is limited by the marked differences in lifespan over which changes in vascular stiffness develop. Studies of gender differences with aging are even more limited in rodents, due to the fact that the estrogen levels never decline even in very old rodents, and they do not go through menopause. It is generally agreed that non-humans primates are the best models to study gender differences with aging, since the changes in hormones and menstruation in old female (OF) monkeys parallel those in older human females. Our previous studies and preliminary data in aging monkeys have demonstrated that the stiffness of the aorta increases with aging and this aging alteration is greater in males than females, and also much greater in the abdominal aorta (AA) vs. the thoracic aorta (TA), which is only partially explained by variance in extracellular matrix (ECM). Here, we will test the novel Hypothesis that intrinsic mechanisms in the vascular smooth muscle cells (VSMCs) as well as alterations in VSMC-ECM interaction also contribute to the increased stiffness of the aorta in older males, particularly the AA, and conversely, contribute to the protection in pre-menopausal females. This Hypothesis is supported by Preliminary Data demonstrating enhanced stiffness of VSMC in culture from old male (OM) aortas and showing that the number of senescent VSMC increases in OM compared to young males (YM), particularly in AA. Specifically, we will test our Hypothesis through two approaches. In the first approach, we will determine how VSMC stiffness and senescence are affected by age and gender using atomic force microscopy (AFM) and also an artificial tissue model. In the second approach, we will determine both in vivo and in vitro how these factors may explain the regional differences in aortic stiffness between TA and AA.